U.S. Democratic presidential candidate Hillary Clinton's campaign said the FBI is investigating whether Russia is behind the hacking and leak of a top adviser's e-mail last week.

Campaign chairman John Podesta said on October 11 that he learned of the investigation over the weekend when he was contacted by the Federal Bureau of Investigation about the breach of his e-mails, some of which were published by WikiLeaks on October 7.

Podesta said he was told the investigation has been incorporated in the FBI's ongoing investigation into the hacking of Democratic organizations by Russian intelligence.

Podesta's disclosure came as the White House said President Barack Obama is considering options for a "proportional response" to verified incidents of Russian hacking.

Podesta says Russia's "interference" in the election and apparent attempt to influence it on behalf of Clinton's Republican opponent Donald Trump should be of "utmost importance" to all Americans.

Podesta would not confirm the authenticity of his leaked e-mails, noting that the Russians have doctored documents in past hacking incidents.

Russia's ambassador to the United States on October 11 denied his country tried to influence the election, including by hacking the e-mails of Clinton staff.